This invention relates to flexures in general and flexures used to connect rotor blades to the rotor of a rotorcraft such as a helicopter, in particular.
It has long been recognized that it is desirable to couple pitch motions of a helicopter rotor blade to flap motions of the blade. The flap angle of a rotor blade is defined as the angle between the rotor blade and the plane of rotation of the rotor. The flap angle is positive in an upward direction and is a direct function of the amount of lift the blade generates. The blade pitch angle or angle of attack is the angle between the free-stream air velocity vector and the rotor blade chord plane. The pitch angle is positive when the leading edge of the rotor blade is pitched upward. Rotor blade lift increases with increasing pitch angle until aerodynamic stall is reached.
Without the use of the present invention or some other device which will achieve the same affect, an increase in pitch angle will produce an increase in flap angle. This results from the fact that the greater lift which results from the increase in the pitch angle forces the blade flap angle to increase. In rotors of the bearingless type, high flap angles are particularly undesirable due to the high stress levels which are accordingly induced in the root ends of the rotor blades. This positive coupling of rotor blade flap angles to the blade pitch angle also causes stability and control problems for the aircraft.
Most helicopters of the prior art have used articulated rotors. Articulated rotor systems are rotors wherein the rotor blade is hinged to the rotor hub. These discrete hinges have usually been provided by hinge pins or the like. Flap hinges are provided so that the rotor blades may flap in and out of the plane of rotation. A lead lag hinge is provided so that the rotor may move to and fro in the plane of rotation. Additionally, bearings are provided which permit the rotor blade to pitch about its longitudinal axis. With advances in technology rotor systems are presently being made without such bearings. In a bearingless rotor system the discrete flap and lead lag hinges of the articulated rotor system are eliminated. The flapping and lead lag motions occur, but do so as a result of elastic deformation of the blade, or flexure.
It has been common to design articulated helicopter rotor systems such that there is negative coupling between the pitch and flap angles. Negative coupling causes the blade pitch angle to decrease with an increase in flap angle. The reduction of blade pitch decreases the amount of lift generated by the blades thus permitting the flap angles to decrease. Negative coupling thus avoids the problems of high stress levels at the root ends of the rotor blades and decreases stability and control problems of the aircraft. The achievement of negative pitch flap coupling has been a problem with the advent of bearingless rotor systems.